1. Field of the Invention
This invention relates generally to computerized process control systems and more particularly to a programmable computer or controller having a program for simulating control circuitry for controlling a plurality of equipment units, all sharing a common control circuit.
2. Description of the Prior Art
Programmable digital computers or controllers for controlling industrial processes have been in use for years. More recently, several special purpose programmable controllers have been developed which function in industrial control systems of the type normally utilizing relay logic to perform required control functions.
Typically, the documentation utilized to show how the relays and their contacts are interconnected to depict the control logic is in the form of an electrical schematic (circuit) diagram. The format of one type of electrical schematic resembles a ladder, commonly referred to as a ladder diagram. The ladder diagram is comprised of two vertical spaced buses each connected to an appropriate power supply terminal (+and -). Horizontal circuit conductors are connected between the vertical buses, giving the circuit the appearance of a ladder.
Each circuit conductor normally includes a relay (coil) as a last element which is preceded by one or more switch elements (e.g. relay contacts) which control the energization and de-energization of the relay to generate an output signal in accordance with continuity established through the horizontal conductor. AND functions can be performed by connecting switch elements in series in a horizontal conductor while OR functions can be performed by connecting switch elements in parallel. Thus, the switch elements can be considered to represent logic elements for performing Boolean logic functions.
In general design practices, after the ladder diagram has been completed, a control system is wired corresponding to that diagram. The switch and relay elements of a circuit conductor may be operated by or operate other switch and relay elements in system. To define these other elements, each element on the diagram is labeled with a reference. With these labels on the diagram, the circuit interconnections can easily be determined by referring to the diagram.
Many industrial processes have several like equipments (e.g. motor drives) which have the same operating parameters. That is, each equipment operates in the same manner so that the control logic for each of the equipments can be the same.
In present day hardwired and programmable controllers, separate control logic circuitry must be implemented to control each of those like equipments because each provides its own distinct output signals and requires its own distinct input signals. Since each equipment requires its own control circuitry, controller cost is increased because of circuit redundancy. In the present invention this redundancy and high cost is overcome in a programmable computerized control system.